Recombinant avirulent Salmonella vaccines are being studied for their ability to express cloned genes of virulence antigens of heterologous pathogens, and for their potential use in the development of mucosal vaccines protective against various infectious diseases including dental caries. However, the mechanism(s) by which these safe, live vector vaccines mediate host responses are still poorly understood. Pattern-recognition receptors (PRRs), including the Toll-like receptors (TLRs), are expressed on/in host cells and recognize distinct molecular patterns associated with microbial components. The recognition of microbial components by TLRs on dendritic cells leads to the induction of an innate immune response and the activation of adaptive immunity. Our findings from immunization studies with a Salmonella vector vaccine expressing the gene encoding the saliva-binding region (SBR) of the adhesion AgI/II of Streptococcus mutans showed that the costimulatory molecules CD80 and CD86 play distinct, as well as redundant roles in mediating mucosal and systemic antibody responses, thus providing evidence that the vector and SBR may be signaling the innate immune system differently, presumably via TLRs. Other evidence suggests that more than one TLR may be involved in mediating and potentiating the immune response to SBR produced by the Salmonella vector. The fundamental hypothesis of our proposed studies is that the interplay between TLRs on various host cells that are engaged following exposure to the Salmonella vector vaccine will influence the outcome of the adaptive immune response to the cloned antigen expressed by the Salmonella. Our studies will focus on defining the regulatory roles of TLRs that mediate the adaptive immune response following mucosal immunization with a complex Salmonella vector vaccine expressing a cloned virulence antigen of S. mutans. The specific aims are; 1) to define the TLRs and the interplay between the TLRs involved in a selective augmentation of mucosal and systemic responses to the cloned antigen of S. mutans expressed by a Salmonella vector vaccine and 2) to determine the role of TLRs on T cells and B cells for an adaptive immune response to a cloned antigen expressed by a Salmonella vector vaccine. In vivo immune response and adoptive transfer models that use TLR knockout mice, as well as in vitro assay systems will be used to address the aims. An understanding of each TLR and the interplay between TLRs involved in mediating the innate and adaptive host responses to a vaccine candidate, and the contribution of TLR signaling in dendritic, T and B cells for the resulting adaptive response to specific microbial antigens will help in the design of improved Salmonella vector vaccines and immunization regimens for the induction of protective immune responses against infectious diseases, including dental caries. PUBLIC HEALTH RELEVANCE: Recombinant attenuated Salmonella vaccines are being developed and tested for their effectiveness in inducing protective mucosal and systemic immune responses to a variety of infectious agents, including the etiologic agents of dental caries and periodontal disease. Live vector vaccines are more effective in inducing mucosal immune responses against microbial pathogens and more economical to produce than vaccines consisting of purified microbial components. Therefore, it is critical to determine the mechanisms involved in host recognition of microbial components of a Salmonella vector vaccine that regulate the nature of the immune response induced to the expressing cloned virulence antigen of a heterologous pathogen, such as mutans streptococci, in order to develop recombinant avirulent Salmonella vaccines that are highly effective in potentiating protective immune responses against mucosal pathogens, including those associated with the oral cavity.